If your relieves on the calipers have been touched then air has entered your braking system causing this problem. This will cause to lose pedal once in a while once it hits an air pocket You have to bleed the brakes in this case.

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There are many things other than brake lines that can cause braking loss. Investigation will reveal the issue. First, check fluid level in the Master Cylinder. If the level is normal, you don't have any broken lines/leaks. If the fluid is full and you have no brake pedal/braking power, chances are the Master Cylinder itself has failed. There are o-rings internal to the M/C that can fail, and when this happens, you will lose pressure to wheel cylinders/calipers and, as a result, lose your brakes.

If the fluid has leaked out, then you need to find out fro where. This is not too difficult normally. Fill the M/C with fluid and have someone press the brake pedal while observing the underside of the vehicle. You should see fluid dripping from somewhere, often near one of the wheels. You can have a leaking caliper/wheel cylinder, brake hose, or metal brake line. Once you determine what part has failed, you can purchase and replace the defective part. Brake calipers, wheel cylinders, and hoses are fairly simple to replace. Brake lines are more difficult, as they require shaping, cutting, and the creation of couplings. After repairs are done, the brake system must be refilled and all air bled from the system or braking will not be satisfactory.

The problem is in the master cylinder. The MC is in effect two cylinders in one, one cylinder for the front brakes and one for the rear (one in front of the other). When the MC gets a bit long in the tooth pressure from one cylinder can leak past it's seal into the other cylinder or into the reseviour.
The MC is leaking internally, but not externally, so you see no signs of leakage and don't lose any fluid as leaked fluid ends up back in the reseviour. Sometimes the symptom will be evident and other times not, on my car the pedal tends to sink sometimes on hot days but is always firm on cooler days(?).
It really is something that does need to be addressed urgently as a matter of safety, because pressure is leaking from one cylinder the pressure in that cylinder won't be high enough. This will affect front to rear brake bias which can have a substantial adverse affect on braking performance, and may cause premature brake lock up at either the front or rear (not sure which end), which could cause loss of control (i.e. massive understeer or oversteer under brakes).
$700 does sound at least a bit steep to supply and fit a new MC, and bleed the brakes. Here in Australia new Accord MCs are around AUD$180 or so.
The pedal should be quite firm with little free play with a braking system in good condition, if yours typically isn't (before you had that sinking feeling) then the brakes probably need attention (bleeding most likely, but could also be a soft rubber hose, causing the hose to bulge under pressure, not good as it may burst).

if you have no indication of a brake fluid leak then you may have a bad master cylinder that has an internal leak that is losing brake fluid and being burnt (sucked into engine)by way of the power brake booster. if you undo the master from the power brake booster and move it ahead enough to see if it is damp were it bolts to the booster then you have a defective master cylinder.

suggest you "look" for fluid loss issue. fluid has to go somewhere right? either a bad rear wheel cylinder.which inside of rear tires will be wet with brake fluid..either passenger side or drivers side. or a bad brake caliper on the front.which will tell on itself by leaking fluid onto the ground when brakes are applied..brake fluid leaking will tell on itself by inspecting the inside of each tire,on the inside of the tires,front to rear. thank you for choosing fixya.com.

check the master cylinder. it can go bad without leaking. the fluid bypasses the internal cups causing a fall in the pedal. also if you broke the lines lose to do calipers you might have air in the system bleed it good. if not i suspect the master cylinder

well if you are losing fluid you definatly have a leak as brake fluid is not a "topping off fluid". you could have a leaking master cylinder either externaly which can be seen on brake booster or it can be leaking into the brake booster filling it with fluid. other than that you also may be leaking from your brake hoses(one hose at each wheel), calipers(front wheels and rear if all wheel disc brakes), wheel cylinders (if you have rear drum brakes), or metal brake lines due to rust and corrosoin (run from master cylinder to each brake hose). please rate 4 thumbs if helpful.

1. What is the Brake Fluid Level?
2. Are you losing any Brake fluid at all? Are there any puddles or noticeable wetness on the ground or on the tires/wheels? With vehicle parked, the reservoir filled (overfilled), and engine running: pump the brakes and check on the ground and back sides (inside portions) of the wheels. Look for any brake fluid leaks.
3. Leaks from Calipers usually means a leak from the caliper piston. The rubber seals will be wet.
Leaks from Drums usually means a leak from the wheel cylinder.
4. In any case of leaks from the wheels - replace the calliper OR wheel cylinder.
For caliper leaks: change the brake pads, and clean the rotors with brake cleaner.
For drum leaks: clean the drum and brake hardware with brake cleaner, and replace the shoes.
5. If no leak is detected from the wheel area's: check under the Master Cylinder for wetness indicating a possible leak.
6. If no leak can be found: you MAY have a rear seal leak in the Master Cylinder which is pumping Brake Fluid into the Brake Booster (that large disk looking thing attached to the fire wall, attached to the Master Cylinder in front).
a. Use a large drip/catch pan under the Van in the area on the Master Cylinder.
b. Disconnect the brake lines from the Master Cylinder using a flair wrench.
c. Remove the two nuts (12 or 13mm?) attaching the Master Cylinder to the Brake Booster. If you have been using a lot of fluid, and have found no leaks, here is a good possibilty that all that brake fluid has collected in the Booster; so when you detach the Master Cylinder from the Brake Booster, all that fluid will come rushing out!
7. If there is Brake Fluid inside the Brake Booster: there is no cleaning that out. Brake Fluid is very caustic to the seals inside the Booster and should be replaced.
8. Of course the Master Cylinder will also have to be replaced.
9. After replacing the Master Cylinder/Brake Booster - make sure you get all that spilled brake fluid off the engine compartment area parts! I use a brake cleaner, then mild soap and then water to rinse. Clean any Brake Fluid off Paint imediatley as it will quickly dissolves finishes and paint.
10. If no leak is detected and the Booster is clear, there may be problems with the Booster One-Way Valve. This is attached to the outside of the Booster and has a hose from an "advance" connected to it. This maybe malfunctioning where you are losing power. Replace this first before deciding to replace the whole booster (in cases of NON-LEAKS ONLY). If that did not work, there may be inner seals inside the Booster that have failed. That means a new Booster.
11. In any of the above cases: Make sure you bleed the Master Cylinder correctly and bleed the brakes (at each affected wheel) correctly. Any air in the brake lines will decrease your braking proficiency or could result in brake failure.

Let me know if this helped or if you have any additional information or questions. Feel free to contact me at FixYa.com!

after removing the wheel, remove the 2 bolts on the brake caliper. place a clamp on the flexy brake line to stop fluid loss. remove the caliper then remove the pads. undo the bleed nipple and push the piston back into the caliper. do up the nipple. undo the front wheel bearings and remove the disk. pack the wheel bearings and fit the front hubs. fit the brake pads then the caliper. remove the clamp check the fluid levels and pump the brakes brakes should be done by a professional because if the fail you can only die once
happy motoring